Using an Oscilloscope

Electricity Lesson 12

Homework

Revise for the exam!

Make revision check lists (lists of definitions, equations, derivations), make sure you know these easy marks in the exam.

Practice questions, past papers, out of the book, go on the website if you need more.

Learning Objectives

To know what an oscilloscope is and how it works.

To know how to measure the pd of alternating current and direct current.

To know how to measure the frequency of an alternating current.

Cathode Ray Oscilloscope

Definition

From the specification book:-

An oscilloscope consists of a specially made electron tube and associated control circuits.

The electron gun emits electrons towards a fluorescent screen light is emitted when electrons hit the screen this is what we see.

Tube Photograph

Electron Gun Photograph

Tube Diagram

H.T. supply

heater supply

y plates

+-phosphor screen

anode

x plates

How does it work?

When no p.d. is applied across the plates the spot on the screen is stationary.

If a pd is applied across the X-plates the beam of electrons is deflected horizontally and the spot moves across.

pd across Y-plates spot moves up and down.

Oscilloscope Controls

The x-plates are connected to a time base circuit which is designed to make the spot move across the screen in a given time then back again much faster. a bit like a trace on a heart monitor.

The y-plates are connected to the Y-input and this causes the spot to move up or down depending on the input pd.

summary

H.T. supply

heater supply

y plates

+-phosphor screen

anode

electron gun produces a beam

of electronslight produced on

the screen by electron beam

a p.d. across the y plates deflects the

trace vertically

a p.d. across the x plates deflects the trace horizontally

x plates

Oscilloscope Controls

The gain sets the scale for the y-axis, normally in volts per cm.

The time base sets the scale for the x-axis, normally in ms per cm.

Recall that frequency can be calculated from the period from the graph using: period

1frequency

Gain and Time-Base Controls

Peak Voltage

Peak p.d. = 3 Divisions x 1.0 mV/div = 3.0 mV

Period & Frequency

period = 4.0 divisions x 1.0 ms/div = 4.0 msfrequency = 1 / periodfrequency = 1 / 0.004 sfrequency = 250 Hz

Self Test